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Dept. of second thoughts: add comments cautioning against using

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ReadOrZeroBuffer to fetch pages from beyond physical EOF.  This would
usually work, but would cause problems for md.c if writes occurred
beyond a segment boundary when the previous segment file hadn't been
fully extended.
This commit is contained in:
Tom Lane 2007-05-02 23:34:48 +00:00
parent 8c3cc86e7b
commit 63735ca815
1 changed files with 7 additions and 2 deletions
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9
src/backend/storage/buffer/bufmgr.c
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@ -8,7 +8,7 @@
* *
* *
* IDENTIFICATION * IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.217 2007/05/02 23:18:03 tgl Exp $ * $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.218 2007/05/02 23:34:48 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@ -21,7 +21,9 @@
* cache we don't read it, but just return a zeroed-out buffer. Useful * cache we don't read it, but just return a zeroed-out buffer. Useful
* when the caller intends to fill the page from scratch, since this * when the caller intends to fill the page from scratch, since this
* saves I/O and avoids unnecessary failure if the page-on-disk has * saves I/O and avoids unnecessary failure if the page-on-disk has
* corrupt page headers. * corrupt page headers. Caution: do not use this to read a page that
* is beyond the relation's current physical EOF; that is likely to
* cause problems in md.c when the page is modified and written out.
* *
* ReleaseBuffer() -- unpin a buffer * ReleaseBuffer() -- unpin a buffer
* *
@ -137,6 +139,9 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
* cache already, it's filled with zeros instead of reading it from * cache already, it's filled with zeros instead of reading it from
* disk. The caller is expected to overwrite the whole buffer, * disk. The caller is expected to overwrite the whole buffer,
* so that the current page contents are not interesting. * so that the current page contents are not interesting.
* Caution: do not use this to read a page that is beyond the relation's
* current physical EOF; that is likely to cause problems in md.c when
* the page is modified and written out. P_NEW is OK, though.
*/ */
Buffer Buffer
ReadOrZeroBuffer(Relation reln, BlockNumber blockNum) ReadOrZeroBuffer(Relation reln, BlockNumber blockNum)